The present invention relates to an ion source apparatus for heating a thermionic cathode by discharge and/or electron bombardment to produce thermoelectrons, ionizing gas molecules by the thermoelectrons, and extracting ions.
In an ion source apparatus supplied to an apparatus, such as an NBI (neutral beam injector), a plasma is produced by gaseous discharge, and ions are extracted from the plasma and accelerated into a high-speed ion beam by an electric field. In a prior art of ion source apparatus using a plurality of linear heating elements or wires as a thermionic cathode, the heating wires may be conducted to emit thermoelectrons, and a gas in a discharge chamber ionized by the thermoelectrons. However, since the thermionic cathode is subjected to evaporation or sputtering, the service life of the thermionic cathode is short. This requires frequent replacement of the thermionic cathode consisting of a plurality of wires, which results in inconvenient and costly operation of the NBI.
In an ion source apparatus using a plate-shaped thermionic cathode, an auxiliary electrode is incorporated to oppose the thermionic cathode. An AC voltage is supplied between the thermionic cathode and the auxiliary electrode so as to produce electron emission current. In such an apparatus, while the thermionic cathode is heated by electron bombardment or discharge, the auxiliary electrode must also be heated. Therefore, when a plasma for extracting ions is to be maintained, power is also consumed for heating the auxiliary electrode.